Kinya Kawamura

Electrical-pulse-induced resistivity modulation in Pt/TiO2−δ/Pt multilayer device related to nanoionics-based neuromorphic function

Resistivity modulation behavior in Pt/TiO2−δ/Pt multilayer devices was investigated in terms of nanoionics-based neuromorphic function. The current relaxation behavior, which corresponds to short-term and long-term memorization in neuromorphic function, was analyzed using electrical pulses. In contrast to the huge difference in ionic conductivity for bulk crystal materials of TiO2−δ and WO3, the difference in the relaxation behavior was small. Rutherford backscattering spectrometry and hydrogen forward scattering spectrometry revealed that the TiO2−δ thin film contained 5.6 at. % of protons. This indicates that the neuromorphic function in TiO2−δ-based devices is caused by extrinsic proton transport, presumably through the grain boundary.

Electrical and structural properties of TiO2−δ thin film with oxygen vacancies prepared by RF magnetron sputtering using oxygen radical

Anatase TiO2−δ thin film was prepared by RF magnetron sputtering using oxygen radical and Ti-metal target. Degrees of the TiO2−δ crystal orientation in the thin film depends of the oxygen gas pressure () in the radical gun. The (004)- and (112)-oriented TiO2−δ thin films crystallized without postannealing have the mixed valence Ti4+/Ti3+ state. The electrical conductivities, which corresponds to n-type oxide semiconductor, is higher in the case of (004)-oriented TiO2−δ thin film containing with high concentration of oxygen vacancy. The donor band of TiO2−δ thin film is observed at ~1.0 eV from the Fermi level (E F). The density-of-state at E F is higher in (004)-oriented TiO2−δ thin film. The above results indicate that the oxygen vacancies can control by changing the of the oxygen radical.

Resonant photoemission and X-ray absorption spectroscopies of lithiated magnetite thin film

Resonant photoemission spectroscopy (RPES) and X-ray absorption spectroscopy (XAS) were used to investigate the effect of lithiation on the electronic structure of Fe3O4 thin film relevant to the operation mechanism of nanoionic devices to enable magnetic property tuning. Comparison of the Fe 2p XAS spectrum for lithiated Fe3O4 (Li–Fe3O4) with that for pristine Fe3O4 clearly demonstrated that the number of Fe2+ ions at octahedral B sites is increased by lithiation. The valence band RPES spectra of Li–Fe3O4 further showed that lithiation increases the density of states near the Fermi level originating Fe2+ ions at octahedral B sites. These findings agree well with the observed decrease in the saturation magnetization in the magnetization–magnetic field (M–H) loop of Li–Fe3O4 thin film, indicating that minority spins (down spins) increase (i.e., total spins decrease) due to lithiation. The variation in the number of Fe2+ ions at B sites is suggested to be an underlying operating mechanism of a nanoionics-based magnetic property tuning device.

Electronic structure of c-axis controlled α-Fe2O3 thin film probed by soft-X-ray spectroscopy

We have prepared c-axis controlled α-Fe2O3 thin films on Al2O3 substrates by RF magnetron sputtering and studied their electronic structure by soft-X-ray spectroscopy. The lattice constant of c-axis increases with increasing film thickness due to the relaxation of lattice mismatch between α-Fe2O3 and Al2O3 and formation of oxygen vacancies. The electrical conductivity is higher in thicker thin film. The valence band consists of t2g- and eg-subbband of Fe 3d state hybridized with O 2p state. The band gaps of ~25 and ~95 nm thicknesses of Fe2O3 thin film are ~1.8 and 1.4 eV, respectively, which correspond to the activation energy of electron conductivity. The above results indicate that the band gap and the conductivity of α-Fe2O3 thin film directly affect the change of the lattice constant of c-axis and formation of oxygen vacancies.